1. Field of the Invention
The present invention relates to a carrier detecting circuit for detecting the level of a received signal, in particular, to a carrier detecting circuit for use with a radio receiver and a communication apparatus having the carrier detecting circuit.
2. Description of the Related Art
In such a carrier detecting circuit, a logarithmic amplifier output signal of an RF analog circuit is A/D converted into a received signal strength indicator RSSI. The RSSI is compared with a predetermined fixed threshold value. When the RSSI exceeds the threshold value, a carrier detection signal is output as a trigger for determining whether the received signal has been properly detected.
However, if a low value is set as the predetermined threshold value, an undesired carrier is detected due to an interference wave of another system, reception noise, and so forth. Thus, the power will be wasted. In addition, a desired signal will be lost. In contrast, if a high value is set as the predetermined threshold value, a signal with a low RSSI cannot be received. Thus, the reception area becomes narrow.
In a first related art reference as Japanese Patent Laid-Open Publication No. Hei-6-303680, a CSMA/CD (Carrier Sense Multiple Access with Collision detection) type remote controlling system using a wired transmission line such as a bus line is disclosed. In the system, the worst value of the reception level of each terminal is calculated. A threshold value is set for each terminal. Thus, a large noise margin is accomplished.
In a second related art reference as Japanese Patent Laid-Open Publication No. Hei-3-6165, a carrier level setting system that connects modems through a public line is disclosed. In this system, a particular signal with a predetermined data transmission output level is transmitted. As the result of measuring the particular signal level in the received side, with a predetermined carrier detection level table according to the signal level, a carrier detection level is automatically determined.
In a third related art reference as Japanese Patent Laid-Open Publication No. Hei-2-243035, a carrier detection level setting system is disclosed. In the system, the carrier detection level of a secondary channel carrier signal for a data modem is set. The data modem sets a frequency band division type secondary channel for transmitting remote control information corresponding to a half duplex communication protocol using a four-wired communication line system. In this system, when the noise level of the secondary channel exceeds the carrier detection level, the carrier detection level is automatically set to a higher level. In contrast, when the signal level of the received signal becomes lower than the carrier detection level, it is automatically set to a lower level. Thus, a communication defect against the fluctuation of the line characteristics can be prevented.
However, in the above-described related art references, since the threshold value for detecting a received signal corresponding to the level of the electric field is fixed, a carrier may be incorrectly detected due to an external noise and/or an undesired radio wave. In addition, because of a margin for preventing an incorrect carrier from being detected, the threshold value is set to a higher level than the proper level. Thus, the reception area is unnecessarily narrowed. In other words, when a carrier is detected with a predetermined threshold value, the carrier is incorrectly detected and the reception area becomes narrow. However, it is difficult to set a threshold value corresponding to the operational environment of the system for use.
In addition, with one cumulating circuit, a threshold value is measured and a carrier is detected using an RSSI signal that represents the level of the electric field of the received signal. Thus, a carrier cannot be detected at high speed and with high reliability.
An object of the present invention is to provide a carrier detecting method that detects a carrier with high performance and that suppresses a carrier from being incorrectly detected and the reception area to be widened. Another object of the present invention is to provide a carrier detecting circuit that operates at high speed and with high reliability.
A first aspect of the present invention is a carrier detecting circuit for detecting the threshold value of the level of a carrier that can be received, comprising a means for adaptively varying the threshold value corresponding to a radio environment of a desired radio wave, a disturbed radio wave, and noise.
A second aspect of the present invention is a carrier detecting circuit for detecting a threshold value of the level of a carrier that can be received, comprising an RSSI block cumulating circuit for cumulating an RSSI signal of which the level of the carrier has been detected and A/D converted so as to calculate an RSSI signal in a low level, an RSSI level raising circuit for raising the RSSI signal calculated by said RSSI block cumulating circuit by a predetermined level, an RSSI continuous cumulating circuit for always cumulating the RSSI signal that has been A/D converted, and a comparing circuit for comparing the threshold value that has been raised by said RSSI continuous cumulating circuit with the output signal of said RSSI continuous cumulating circuit so as to determine whether or not a carrier is present.
A third aspect of the present invention is a communication apparatus for detecting a threshold value of the level of a carrier that can be obtained from a received signal, comprising an RSSI block cumulating circuit for cumulating an RSSI signal of which the level of the carrier has been detected and A/D converted so as to calculate an RSSI signal in a low level, an RSSI level raising circuit for raising the RSSI signal calculated by said RSSI block cumulating circuit by a predetermined level, an RSSI continuous cumulating circuit for always cumulating the RSSI signal that has been A/D converted, and a comparing circuit for comparing the threshold value that has been raised by said RSSI continuous cumulating circuit with the output signal of said RSSI continuous cumulating circuit so as to determine whether or not a carrier is present.
According to the present invention, in a region (time period) of which a burst signal is not received, RSSI is cumulated for a predetermined region (time period). Thus, the fluctuation of the noise level is smoothed. An RSSI signal in a low level is detected by an RSSI block cumulating means (101). The level of the output signal of the RSSI block cumulating means (101) is raised by an RSSI level raising means (103) for a predetermined value so as to suppress a carrier from being incorrectly detected. The output signal of the RSSI level raising means (103) is used as a threshold value.
The same fluctuation of the output signal as the clock timing of the A/D conversion is obtained by an RSSI continuous cumulating means (102). The RSSI is always cumulated by the RSSI continuous cumulating means (102) so as to smoothen the fluctuation of the noise level. The output signal of the RSSI continuous cumulating means (102) is compared with the threshold value by a comparing means (104) so as to determine whether or not a carrier is present.
According to the present invention, the threshold value for detecting a carrier is measured in a guard time period between a transmission burst and a reception burst. Thus, since the threshold value is adaptively varied, the communication distance for detecting a carrier can be substantially prolonged. Consequently, the communication area can be widened. In addition, a carrier can be prevented from being incorrectly detected against noise and an interference signal of another system.
In addition, with the cumulating system according to the present invention, since an output signal with the same fluctuation as the clock timing of the A/D conversion is obtained, the fluctuation of the noise level can be smoothed. In addition, a carrier can be detected at high speed. Thus, a synchronous preamble added at the beginning of data is not wasted. In this case, the radio environment means the reception level of a carrier in a desired reception band (for example, the level of a radio wave that fluctuates time by time due to a multi-path or fading and the state of an insertion of an undesired radio wave). In other words, the radio environment means fluctuation states of the level of a desired radio wave, the level of a disturbed radio wave, and the level of noise.